The performance of piezoelectric actuators and sensors is dependent on the microstructural properties of the piezoelectric materials. Commercially available piezoelectric materials, including PZT, PLZT, and PMN-PT, typically exhibit good piezoelectric response but poor fatigue life. Attempts have been made in the past to improve the microstructure of piezoelectric materials using fine-grained precursors with conventional sintering techniques. Although such attempts at microstructural improvement have enhanced some piezoelectric performance characteristics, fatigue life remains low because conventionally sintered ceramics typically have densities of only 90-95% of their theoretical values. U.S. Pat. Nos. 5,595,677 and 5,607,632 present new piezoelectric materials with improved grain orientation, greater density, and extended fatigue life and a process for forming these piezoelectric materials. Those new materials comprise a family of Nb-doped PLZT piezoelectric ceramics.
The PLZT ceramics, according to the '677 and '632 patents, are fabricated using PbO, TiO.sub.2, ZrO.sub.2, and La.sub.2 O.sub.3 powders as starting materials in a hot forging technique to produce a PLZT piezoelectric ceramic having the general formula (Pb.sub.1-x La.sub.x) (Zr.sub.y Ti.sub.1-y).sub.1-(x/4) O.sub.3. In the steps of the fabrication method, ZrO.sub.2 and TiO.sub.2 powders are mixed at a molar ratio of y/(1-y), calcined at approximately 1300.degree. C.-1500.degree. C., and ball milled in acetone. After milling, the acetone is evaporated to produce a dry powder. The mixture of ZrO.sub.2 and TiO.sub.2 is then combined with appropriate amounts of the PbO and La.sub.2 O.sub.3 powders plus Nb.sub.2 O.sub.5 added to provide 0.5-1.5% Nb.sup.5+ (mole %) as a dopant. The new mixture is ball milled in acetone, evaporated to a dry powder, calcined at approximately 700.degree. C.-850.degree. C., and sifted to obtain a particle size of approximately 0.3-2.0 .mu.m. The final PLZT powder is then formed into the desired shape by cold pressing (typically at 15,000-25,000 pounds, for example) followed by sintering at approximately 1000.degree. C.-1150.degree. C. in oxygen. Further densification may be accomplished by heating to approximately 1175.degree. C.-1275.degree. C. at 800-1200 psi to achieve a density of at least 97% (preferably at least 98.5%) of the material's theoretical maximum density.
The PLZT ceramic materials prepared by the process of the '677 and '632 patent displayed a strong dielectric permittivity maximum at approximately 155.degree. C.-165.degree. C., above and below which the permittivity drops rapidly. The ceramic is a polar dielectric below about 112.degree. C.-125.degree. C. with a stable net spontaneous polarization, P.sub.r, and a well-defined polarization hysteresis loop (P versus bipolar electric field). A distinguishing feature of PLZT ceramics made by the process described in U.S. Pat. Nos. 5,595,677 and 5,607,632, compared with other piezoelectric ceramics, is a reduced strain hysteresis with the application of a unipolar electric field and a breakdown strength of greater than 25 kV/cm (typically 25-30 kV/cm), which is well above the typical operating electric field strengths that are normally used for piezoelectric actuation. Furthermore, the linear piezoelectric coefficient (d.sub.33), the maximum strain (%), and the fatigue life (cycles) of the Nb-doped PLZT piezolectric ceramics is significantly improved over prior known piezoelectric materials. These materials are designed to operate between -50 and +100.degree. C., having high strain properties (0.20%) in this temperature range. However, since these materials have a Curie temperature (Tc) of about 115.degree. C., they can not be used above about 100.degree. C. As a result, they fail to demonstrate suitable piezoelectric properties when operated at higher temperature situations (approximately 200.degree. C.) and therefore can not be used in aircraft, some aerospace and engine environments.
A principal object of the present invention is to provide piezoelectric materials which have improved performance at higher temperatures. Additionally, the Nb/Ta-doped PLZT piezoelectric ceramic incorporating features of the invention have a high density and fine grain microstructure. An advantage of these new PLZT piezoelectric ceramics is that they exhibit a strain rate of approximately 0.17% at .about.200.degree. C. while still maintaining a d.sub.33 of about 800 pC/N.